I have only seen pictures or these things on cameras ******* on ebay and had heard about them. What it is is a set of contacts that attach next to the shutter wind key, one being fixed position and the other positioned so that the wing of the wind key pushes it into the other contact and both contacts are connected to the flash. These work with focal plane ,fp, flash bulbs only.
There is another type that puts a set of contacts on the mirror that triggers the flash when the mirror reaches the full up position but it requires more modification of the camera to install.

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